991 Fixed Filters

Moog 991 Rear

Introduction In August 2020 I bought a set of Behringer System 55 modules, as my first journey into Moog modular. My plan is to recreate the Klaus Schulze IIIP synthesizer and Dual Sequencer setup that Klaus bought on 22nd December 1975, and used successfully for many years. Whilst the Behringer modules cover most of the Moog modules I need, there are a few additional modules and changes I have made.

The 991 Module This is a replica of the original Moog design from the IIIP modular, which is a very simple pair of fixed filters (single pole -6dB Low Pass and High Pass) and an attenuator. There are five filter positions for each filter and the extra attenuator is really handy.

991 Artwork

It was dropped from the System 55 as customers probably made little use of it. However its a nice alternative to the 923 module, as the 991 fixed positions require no dialing in of values.

The three circuits are totally passive with no power needed. There are five different value polyester capacitors in each filter with a 6 way rotary switch selecting each capacitor in turn, with a final bypass position at step 6.

The cutoff frequency of the filters is the same;

  • Low Pass¬† = 16Hz, 41Hz,159Hz, 482Hz, 1591Hz
  • High Pass = 16Hz, 41Hz,159Hz, 482Hz, 1591Hz

AMSynths 991 A new PCB has been designed which holds the switches, pot, jacks sockets and filter capacitors. No power inlet is required as the Behringer 55 modules use earthed jack sockets, which makes the passive circuits work. I have use Tropical Fish capacitors and carbon resistors for an original sound and look!

Outcome The PCB was ordered in early December once I had tracked down the right Tropical Fish capacitors and measured the lead spacing. The prototype works perfectly and is best used as a way of filtering white noise rather than oscillator sounds. A small production run was manufactured in Spring 2021.

Copyright AMSynths 2019